1. Technical Field
The invention is related to a suspension assembly for a direct access storage device such as a magnetic disk drive having a magnetic head and in particular to a head-gimbal assembly (HGA) thereof.
2. Background Art
The sub-micron spacing between the magnetic recording head and spinning disk in a direct access storage device (DASD) such as a hard disk drive is maintained by an air bearing and must remain within a narrow specified range for the DASD to function as designed. For this purpose, the generally flat bottom surface of the magnetic head must be held at some precisely specified orientation relative to (e.g., parallel to) the surface of the spinning disk.
Referring to FIGS. 1 and 2, a head-gimbal assembly (HGA) 10 includes a base plate 20 with a boss 25. The base plate 20 is installed in a DASD by fastening it to a positioning apparatus or structure 30 of the DASD which holds the base plate 20 parallel to the spinning disk 35 below. A magnetic head 40 is coupled to the base plate 20 by a gimbal 45 and load beam 50. The magnetic head 40 is bonded to the gimbal 45 by an adhesive or glue 55. Deflection of the gimbal 45 toward the load beam 50 is limited by a dimple 60 which permits some pitch and roll of the elastic gimbal 45 relative to the load beam 50. Ideally, the magnetic head 40 is parallel to the disk 35 as long as the base plate 20 is parallel to the disk 35. This ideal state is realized, however, only if the gimbal 45 and the load beam 50 are both perfectly straight so that the magnetic head 40 is parallel to the base plate 20. In practice, the mass production of the HGA 10 is characterized by a statistical distribution of deviations of the magnetic head 40 from the desired parallel orientation relative to the base plate 20. For example, as shown in FIG. 2, the load beam 50 may be twisted, so that the head 40 is held at a slight roll angle relative to the base plate 20.
The HGA 10 is installed in a DASD including the disk 35 and the positioning apparatus 30. The disk 35 begins spinning at the specified rate to form an air bearing under head 40 which lifts the head 40 slightly above the surface of the disk 35. During DASD assembly, the load beam 50 is deflected from its equilibrium position (dashed line of FIG. 1) through an angle A, thereby generating a force proportional to this deflection which presses the head 40 against the air bearing over the disk 35. If there is a slight twist in the load beam 50 (as illustrate in FIG. 2), this force rotates the head 40 to be almost parallel to the disk 35 as shown in FIG. 3, thereby elastically deforming the gimbal 45. However, the air bearing between the disk 35 and head 40 permits the head 40 to rotate slightly out of the design orientation in response to the elastic moment exerted by the gimbal 45 (i.e., the moment opposing the elastic deformation). This situation is indicated in FIG. 3 by the force vectors of different magnitudes exerted by the air bearing near opposite edges of the head 40. (Each force vector depicted in the drawing represents the integrated pressure under each one of the slider rails 41, 42.) As a result, the gap between the head 40 and disk 35 deforms by a small angle in the direction of the applied moment. In many cases, this angular deformation is sufficient to degrade the ability of the head 40 to magnetically read or write on the disk 35.
Such degradation has seemed to be unavoidable because it has not been possible to form perfectly shaped load beams and gimbals in mass production of HGAs. Accordingly, there is a need to provide better head-to-disk alignment unaffected by structural imperfections in the suspension, such as the twist in the load beam 50 illustrated in FIG. 2.